## [结合组合与继承](https://lingcoder.gitee.io/onjava8/#/book/08-Reuse?id=%e7%bb%93%e5%90%88%e7%bb%84%e5%90%88%e4%b8%8e%e7%bb%a7%e6%89%bf) 你将经常同时使用组合和继承。下面的例子展示了使用继承和组合创建类，以及必要的构造函数初始化: ~~~ // reuse/PlaceSetting.java // (c)2017 MindView LLC: see Copyright.txt // We make no guarantees that this code is fit for any purpose. // Visit http://OnJava8.com for more book information. // Combining composition & inheritance class Plate { Plate(int i) { System.out.println("Plate constructor"); } } class DinnerPlate extends Plate { DinnerPlate(int i) { super(i); System.out.println("DinnerPlate constructor"); } } class Utensil { Utensil(int i) { System.out.println("Utensil constructor"); } } class Spoon extends Utensil { Spoon(int i) { super(i); System.out.println("Spoon constructor"); } } class Fork extends Utensil { Fork(int i) { super(i); System.out.println("Fork constructor"); } } class Knife extends Utensil { Knife(int i) { super(i); System.out.println("Knife constructor"); } } // A cultural way of doing something: class Custom { Custom(int i) { System.out.println("Custom constructor"); } } public class PlaceSetting extends Custom { private Spoon sp; private Fork frk; private Knife kn; private DinnerPlate pl; public PlaceSetting(int i) { super(i + 1); sp = new Spoon(i + 2); frk = new Fork(i + 3); kn = new Knife(i + 4); pl = new DinnerPlate(i + 5); System.out.println("PlaceSetting constructor"); } public static void main(String[] args) { PlaceSetting x = new PlaceSetting(9); } } /* Output: Custom constructor Utensil constructor Spoon constructor Utensil constructor Fork constructor Utensil constructor Knife constructor Plate constructor DinnerPlate constructor PlaceSetting constructor */ ~~~ 尽管编译器强制你初始化基类，并要求你在构造函数的开头就初始化基类，但它并不监视你以确保你初始化了成员对象。注意类是如何干净地分离的。你甚至不需要方法重用代码的源代码。你最多只导入一个包。(这对于继承和组合都是正确的。)